Three-frequency nuclear quadrupole resonance (NQR)

ABSTRACT

This invention involves an apparatus to measure the nuclear quadrupole (NQR) response of a specimen using three frequencies. Three-frequency NQR involves excitation of at least two transitions that causes an observed signal at a third transition frequency. Thus, the transition excited and detected is not irradiated at all. This reduces undesirable interfering signals due to the excitation, for example as a result of acoustic ringing and/or tank circuit ring-down, since the excitation is not applied at the frequency that is detected. This invention will be particularly useful to detect substances selected from the group consisting of explosives and narcotics using nitrogen NQR.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to nuclear quadrupole resonance(NQR). More specifically, the present invention relates to theexcitation of two frequencies to create the desired signal at a thirdfrequency, and also the method and techniques for the creation of spinechoes and other signals analogous to those obtained with standardmultipulse single-frequency NQR methods.

[0003] 2. Description of the Related Art

[0004] Nuclear quadrupole resonance (NQR) is a technique for detectingtarget specimens containing sub-kilogram quantities of narcotics andexplosives having quadrupolar nuclei. Such substances includenitrogenous or chlorine-containing explosives and narcotics. Basically,quadrupolar nuclei will exhibit nuclear quadrupole resonance—a change inthe angle of nuclear spin with respect to its quantization axis when itis excited by radio frequency (RF) radiation pulses at a particularfrequency. In the better-known nuclear magnetic resonance (NMR), thequantization axis is determined externally by the direction of theapplied magnetic field. In NQR, the quantization axis is determined bymolecular parameters. As with NMR, different chemicals require pulses ofdifferent frequencies (i.e., different nuclear quadrupole resonancefrequencies) to cause precession in nuclei. A device used to detectmagnetic or NQR resonance in the quadrupolar nuclei of a target specimenis tuned to emit pulses at the frequency corresponding to the resonancefrequency of the nuclei desired to be detected. A typical NQRexcitation/detection circuit consists of an inductor capacitively tunedto the NQR frequency and nominally matched to the impedance of atransmitter or receiver by another capacitor or inductor. In thisregard, the present invention is related to other methods of NQRdetection as taught in U.S. Pat. No. 5,206,592 issued Apr. 27, 1993 toBuess et al. for DETECTION OF EXPLOSIVES BY NUCLEAR QUADRUPOLERESONANCE, and U.S. Pat. No. 5,233,300 issued Aug. 3, 1993 to Buess etal. for DETECTION OF EXPLOSIVE AND NARCOTICS BY LOW POWER LARGE SAMPLEVOLUME NUCLEAR QUADRUPOLE RESONANCE, and U.S. Pat. No. 5,365,171 issuedNov. 15, 1994 to Buess et al. for REMOVING THE EFFECTS OF ACOUSTICRINGING AND REDUCING THE TEMPERATURE EFFECTS IN THE DETECTION OFEXPLOSIVES BY NQR, and U.S. Pat. No. 5,608,321 issued Mar. 4, 1997 toGarroway et al. for A MEANS FOR DETECTING EXPLOSIVES AND NARCOTICS BYSTOCHASTIC NUCLEAR QUADRUPOLE RESONANCE (NQR), and U.S. Pat. No.5,804,967 issued Sep. 8, 1998 to Miller et al. for A MEANS FORGENERATING SHORT RF PULSES WITH RAPID DETECTOR RECOVERY IN STOCHASTICMAGNETIC RESONANCE, and U.S. Pat. No. 6,242,918 issued Jun. 5, 2001 toHepp et al. for APPARATUS AND METHOD FOR REDUCING THE RECOVERY PERIOD OFA PROBE IN PULSED NUCLEAR QUADRUPOLE RESONANCE AND NUCLEAR MAGNETICRESONANCE DETECTION SYSTEMS BY VARYING THE IMPEDANCE OF A LOAD TO REDUCETOTAL Q FACTOR, and U.S. Pat. No. 6,054,856 issued Apr. 25, 2000 toSuits et al. for MAGNETIC RESONANCE DETECTION COIL THAT IS IMMUNE TOENVIRONMENTAL NOISE, and U.S. Pat. No. 6,104,190 issued Aug. 15, 2000 toBuess et al. for MEANS FOR DETECTING NITRAMINE EXPLOSIVES BY ¹⁴N NQR OFNITRO GROUPS, all of which are incorporated by reference herein.

[0005] It is common to detect a magnetic resonance signal by placing asample to be measured in a tuned, electronically resonant tank circuit.Then, the response of the tank circuit to the electromotive forceproduced by nuclear or electronic spins in the sample is measured. WithNuclear Magnetic Resonance (NMR) or Nuclear Quadrupole Resonance (NQR),the sample is placed in or near an inductor, commonly referred to as acoil, that detects AC magnetic fields.

[0006] The inductance of the coil is tuned with a parallel and/or seriescapacitance to make the circuit electrically resonant at the measurementfrequency. One or more additional reactive impedances (inductors orcapacitors) are typically added to adjust the resistive impedance atresonance to a particular value which optimizes the detectionsensitivity.

[0007] Although the NQR detection technique works reasonably well insome circumstances, one of the challenges that NQR faces is that whenapplying the RF magnetic field needed to detect NQR, one can createfalse signals at or near the NQR frequency. For example, acousticvibrations are created in certain magnetic metals. In turn, as themagnetic domains in the metal vibrate, they induce a signal back in theNQR receiver coil at essentially the same frequency as the drivingfrequency. For example, if one has a suitcase with chrome trimmings, thedetected signal may indicate that there are explosives in the suitcasewhen in fact it is acoustic ringing from the chrome that is observed.

[0008] A multiple-frequency technique would eliminate the false alarmsdue to acoustic ringing. For example, for a spin-1 nucleus such as ¹⁴N,the three transition frequencies between the levels are discrete. Thebasic concept is to use two of the frequencies to excite the thirdtransition, and to then detect the signal from this third transition.This avoids any interference from acoustic ringing since the RF is notapplied at the frequency that is detected. The method disclosed here isthe first direct observation of a NQR transition near a frequency thathas not been used for irradiation.

[0009] There is another advantage to not irradiating at the observationfrequency: the probe recovery time is greatly reduced. Following a highpower RF pulse one has to wait for the energy stored in the coil todissipate before signal can be detected. This recovery time is typicallyabout 20 coil ringdown time constants. The ringdown time is proportionalto the coil quality factor, Q, and inversely proportional to thefrequency. At low frequency, a substantial portion of the signal may belost during the recovery time. In three-frequency NQR, the energy storedin the observation coil is limited to that which may leak in from theexcitation at other frequencies. Therefore, improved sensitivity isexpected with three-frequency NQR at low frequencies when the signallifetime is short.

[0010] The nuclear wave function evolves under a Hamiltonian consistingof the large time-independent quadrupole term and the much smallertime-dependent terms corresponding to the alternating RF magnetic fieldsapplied at two of the three characteristic NQR frequencies. For example,with frequencies for spin-1 nuclei and the principal axes frame (x, y,z) of the electric field gradient tensor at the quadrupolar nucleus, thequadrupolar Hamiltonian H_(Q) is H_(Q)=e²qQ[(3I_(z) ²−I²)+η(I_(x)²−I_(y) ²)]/4 where I is the nuclear angular momentum operator, η theasymmetry parameter of the electric field gradient, q the fieldgradient, and Q the quadrupole moment of the nucleus. The transitionfrequencies between the eigenfunctions of H_(Q) are Ψ_(±)=(3±η)e²qQ/4

and ω₀=ηe²qQ/2

. Here, it is assumed η≠0 or 1 to avoid degenerate energy levels andtransition frequencies which complicate the calculation. To be concise,only the observation at ω₊ arising from irradiation at ω and ω₀ is heretreated in detail, but similar results are expected for otherthree-frequency combinations.

[0011] For a spin-1 nucleus each transition is allowed under a differentorientation of the applied field in the principal axes frame (i.e.,<+|I|−>=<+|I|−>, <0|I_(y)|−>=<0|I|−>, and <+|I|0>=<+|I_(x)|0>).Therefore, for a single crystal, the most efficient NQR excitation anddetection occur when an RF magnetic field of frequency ω₀ is appliedalong the z-direction, that of ω⁻ along y, and the ω₊ detection coilsare sensitive to magnetization oscillating along x. Similarly, for apowder sample it can be shown that the maximum signal is obtained if thetwo RF magnetic fields are mutually perpendicular to one another in thelaboratory frame (x′, y′, z′). The received ‘three-frequency’ NQR signalthen arises from a magnetization which is orthogonal to both the appliedRF fields.

[0012] The Hamiltonian for the interaction of the nucleus with an RFpulse of magnetic field strength B¹⁻ and frequency ω⁻ applied along thex′-axis is H¹⁻=−

γ_(N)B¹⁻I_(x′)cos ω⁻t and with an RF pulse of strength B₁₀ and frequencyω₀ along y′ is H₁₀=−

γ_(N)B₁₀I_(y′)cos ω₀t. (γ_(N) is the gyromagnetic ratio of the nucleus.)The lab frame operators can be expressed as

I _(x′)=(cos α cos β cos γ−sin α sin γ)I_(x)+(sin α cos β cos γ+cos αsin γ)I_(y)−sin β cos γI_(z)I_(y′)−(−cos α cos β sin β−sin α cosγ)I_(x)+(−sin α cos β sin γ+cos α cos γ)I_(y)+sin β sin γI_(z)  (1)

[0013] where α, β, and γ are Euler angles describing the relativeorientation of the principal axes and lab frames. If it is assumed eachRF pulse only excites one transition, H₁₀ and H¹⁻ can be simplified as

H¹⁻=−

γ_(N)B¹⁻(sin α cos β cos γ+cos α sin γ)I_(y) cos ω⁻t≡−

Ω¹⁻I_(y) cos ω⁻t H₁₀=−

γ_(N)B₁₀(sin β sin γ)I_(z) cos ω₀t≡−

Ω₁₀I_(z) cos ω₀t  (2)

[0014] where the dependence on crystal orientation is now containedimplicitly in the newly defined terms Ω₁₀ and Ω¹⁻, the effectiveRF-nutation rates.

[0015] Using the above Hamiltonians, the wave function |Ψ(t)> is foundafter a single RF pulse of duration t_(p) at a frequency ω₀, can bewritten as a simple rotation of the original wave function around z,|Ψ(t)>=e^(−iH) ^(_(Q))

t/

e^(il) ^(_(z)) ^(θ)|ΨI0)>, where θ=Ω₁₀ t_(p)/2. Similarly an RF pulse atthe frequency ω⁻ is equivalent to a rotation about y by Ω¹⁻ t_(p)/2.Furthermore, the wave function after simultaneous irradiation at ω⁻ andω₀ can be shown to be equivalent to a rotation about an axis in the y-zplane, rotated from the z-axis by an angle ξ, where tan ξ=Ω¹⁻/Ω₁₀. Thatis |Ψ(t)>=e^(−iH)

t/

e^(i(cos ξl) ^(_(z)) +sin ξI ^(_(y))

)

|Ψ(0)>, where

={square root}{square root over ((Ω₁₀ ²+Ω¹⁻ ²))}t_(p)/2 is the angle ofrotation defined by the effective RF-field generated by the twoorthogonal RF fields. For notational simplicity, the above cases assumethat all pulses are referenced to zero phase. The effect of includingnon-zero phases is simply the addition of the phases of the ω⁻ pulse andthe ω₀ pulse to the final phase of the signal.

[0016] Consider serial irradiation, where irradiation at the twofrequencies occurs at different times. Using the above operators andstarting from thermal equilibrium, it is found that a pulse of lengtht_(p) ^(a) at ω⁻ followed by a pulse of length t_(p) ^(b) at ω₀ with adelay of τ between the pulses results in an expectation valueoscillating at ω₊ given by $\begin{matrix}{{\langle{I_{x}(t)}\rangle} = {( {N_{0}^{0} - N_{-}^{0}} ){\sin ( {\Omega_{1 -}t_{p}^{a}} )}{\sin ( \frac{\Omega_{10}t_{p}^{b}}{2} )}{\sin ( {{\omega_{+}( {t + t_{p}^{b}} )} + {\omega_{-}( {\tau + t_{p}^{a}} )}} )}}} & (3)\end{matrix}$

[0017] where t is the time after the end of the second pulse. N⁻ ⁰ andN₀ ⁰ are the thermal populations of the eigenstates |−> and |0>, so thatthe amplitude of the signal depends on the initial difference in thepopulations connected by the first transition excited. Note that for asingle crystal the maximum signal occurs when the first pulse induces anutation angle of Ω¹⁻t_(p) ^(a)=π/2 and the second pulse has Ω₁₀t_(p)^(b)=π, or for a crystal oriented for the most efficient excitationγ_(N)B¹⁻t_(p) ^(a)=π/2 and γ_(N)B₁₀t_(p) ^(b)=π. For a powder, theobserved signal is proportional to the average over all possible crystaldirections of the nuclear spin angular momentum projected along the axisof the detection coil. Usually no signal at ω₊ is observed usingreceiver coils oriented in the x′-y′ plane. With a detection coil alongthe z′-axis the signal is proportional to $\begin{matrix}{S_{z^{\prime}} \propto {\int_{0}^{2\pi}\quad {{\alpha}{\int_{0}^{\pi}\quad {{\beta}\quad \sin \quad \beta {\int_{0}^{2\pi}\quad {{{\gamma ( {\cos \quad \alpha \quad \sin \quad \beta \frac{{\langle{I_{x}( {\alpha,\beta,\gamma} )}\rangle}}{t}} )}}.}}}}}}} & (4)\end{matrix}$

[0018] Performing the powder average numerically, it is found that themaximum attainable signal occurs when γ_(N)B¹⁻t_(p) ^(a)=2.13 rad andγ_(N)B₁₀t_(p) ^(b)=4.26 rad rather than π/2 and π (the aligned singlecrystal results) because randomly oriented crystals will experience RFpulses reduced by the angular factors in ω¹⁻ and Ω₁₀ (see Eq. 2). Thisbehavior mimics that seen in single frequency NQR where the maximumsignal for a powder occurs at a nutation angle of 2.08 rad, a thirdlonger than the nutation angle needed (π/2) for a properly orientedsingle crystal. For a powder, the three-frequency maximum signal size is$\frac{2( {1 - {\eta/3}} )}{3( {1 + {\eta/3}} )} = \frac{2\omega_{-}}{3\omega_{+}}$

[0019] of the maximum signal of a single-frequency NQR experiment at ω₊.

[0020] Similarly, simultaneous irradiation of the sample at •⁻ and at ω₀results in an expectation value oscillating at ω₊ such that

<I _(x)(t))=sin 2ξ(1−cos

)sin(w ₊ t)×[(N ₀ ⁰ −N ₊ ⁰)(cos²ξ+sin²ξ cos

)−(N ⁻ ⁰ −N ₊ ⁰)(1+cos

)].  (5)

[0021] From examination of the geometrical terms, a maximum signal for asingle crystal occurs when ξ=π/8 and

=π, or for a crystal oriented for the most efficient excitationB¹⁻/B₁₀=tan(π/8) and γ_(N)B₁₀t_(p)=2π cos(π/8). Using <I_(x)(t)> of Eqn.5, numerically integrate to find the powder-averaged signal in thedetection coil oriented along z′. Again no signal at ω₊ is observedusing receiver coils oriented in the x′-y′ plane. Although the signal isdependent in a complicated manner on the thermal populations, andtherefore η, the parameters which give a maximum signal depend onlyslightly on η(B¹⁻/B₁₀≈tan(π/8) and γ_(N)B₁₀t_(p)≈7.6 rad, approximatelya third longer than 2π cos(π/8)). The maximum signal ranges from 67% to53% (0<72<1) of the corresponding maximum signal for a single-frequencyexperiment at ω₊.

[0022] In practice, there is a small distribution of quadrupoleinteractions within the sample so that H_(Q)=H_(Q) ⁰+ΔH_(Q). A secondsimultaneous pulse applied at time τ after an initial simultaneous pulsecan undo the dephasing caused by this distribution so that a spin-echois formed at a time t=τ after the end of the second pulse (see FIG. 5).Assuming ΔH_(Q) is small enough that it has a negligible effect on thetime evolution during applied pulses (i.e., ΔH_(Q)<<H¹⁻, H₁₀), it isfound |Ψ(t)>=e^(−iH) ^(_(Q)) ⁰ ^((t+t) ^(_(p)) ^(b) ^(τ+t) ^(_(p)) ^(a)

)/

e^(−iΔH) ^(_(Q))

t/

e^(iK) ^(b) e^(−iΔH) ^(_(Q))

τ/

e^(iK) ^(a) |Ψ(0)>, where K^(a)=(cos ξ^(a)I_(z)+sin ξ^(a)I_(y))

^(a) corresponds to the first simultaneous pulse of length t_(p) ^(a)and K^(b)=(cos ξ^(b)I_(z)+sin ξ^(b)I_(y))

^(b) to the second pulse of length t_(p) ^(b). The refocused signal atω₊ is then

<I_(x)(t)>=−sin 2μ^(a)(1−cos

^(a))sin(ω₊(t+t _(p) ^(b) +τ+t _(p) ^(a))+Ωω₊(t−τ)+2Δφ)×[(N₀ ⁰ −N ₊⁰)(cos²ξ^(a)+sin²ξ^(a) cos

^(a))−(N ⁻ ¹ −N ₊ ⁰)(1+cos

^(a))]×sin²ξ^(b) cos²ξ^(b)(1−cos

^(b))²  (6)

[0023] where Δω₊ describes the distribution in ω₊ and Δφ≡Δφ+Δφ₀. (Δφ isthe phase difference between the first and second pulse of ω⁻ and Δφ₀the phase difference for the ω₀ pulses.) For a single crystal, thedistribution due to ΔH_(Q) is completely refocused and the largest echooccurs when ξ^(a)=π/8, ξ^(b)=π/4, and

^(a)=

^(b)=π (or for a crystal oriented for the most efficient excitationγ_(N)B^(a) ₁₀t_(p) ^(a)=2π cos(π/8), γ_(N)B^(b) ₁₀t_(p) ^(b)=2πcos(π/4)). Through numerical integration, it is found that for a powderthe echo is a maximum when B^(a) ¹⁻/B^(a) ₁₀≈tan(π/8), B^(b) ¹⁻/B^(b)₁₀≈tan(π/4), γ_(N)B^(a) ₁₀t_(p) ^(a)≈7.4 rad, and γ_(N)B^(b) ₁₀t_(p)^(b)=5.7 rad. The maximum signal ranges from 48% to 41% (0<η<1) of thecorresponding maximum signal for a single-frequency resonant experimentat ω₊, or approximately 75% of the signal following the first pulse isrefocused for a powder sample. When deriving Eqn. (6) there are severalterms not included that depend on the precise nature of the quadrupolefield distribution and may give rise to echoes at times other than att=τ, as discussed by Grechishkin.

SUMMARY OF THE INVENTION

[0024] The three eigenstates for spin-1 nuclei under a quadrupolarHamiltonian and the corresponding three possible transition frequenciesare shown in FIG. 1. In conventional NQR, only one of those transitionsis irradiated. The resulting nuclear magnetization oscillates at thefrequency of the irradiated transition and is observed. Three-frequencyNQR involves excitation of at least two transitions that causes anobserved signal at the third transition frequency. Two transitions areirradiated to create an oscillating signal from the third. Similartechniques have been used in NMR to create multiple-quantum coherence,but there the excited transition is forbidden and not directlyobservable at its own frequency.

[0025] The ability to detect a signal at a frequency different from theirradiation frequency gives three-frequency NQR some important potentialadvantages over single-frequency NQR. Interfering signals from resonantacoustic ringing (magnetostriction) of certain metals at the irradiationfrequency could be eliminated. In addition the receiver dead-time, thetime after sample irradiation in which the receiver's response to thetransmitted pulses obscures the initial NQR signal, should be reducedsince no RF energy is applied at the frequency of the receiver. Thisreduction of dead-time would be especially important for those materialsin which the free induction time (T₂*) is short.

[0026] It is an object of the present invention to provide amultiple-frequency technique of nuclear quadrupole resonance.

[0027] A further object of the present invention is to provide a systemfor detecting explosives and narcotics by nuclear quadrupole resonance.

[0028] Additional objects and advantages of the invention will be setforth in the description which follows, and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

[0029] Objects of the present invention are achieved by using twofrequencies to excite the third transition, and then to detect thesignal from this third transition. This avoids any interference fromacoustic ringing since the RF magnetic field is not applied at thefrequency that is detected. The method disclosed here involves thedirect observation of a NQR transition near a frequency that has notbeen used for irradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will become more fully understood from thedetailed description given herein and the accompanying drawings whichare given by way of illustration only, and thus are not limitative ofthe present invention and wherein:

[0031]FIG. 1 illustrates energy levels determined by a quadrupolarHamiltonian of nuclear spin I=1.

[0032]FIG. 2 illustrates a block diagram of the NQR system for thepresent invention.

[0033]FIG. 3 illustrates a schematic of the three-frequency NQR system.

[0034]FIG. 4 illustrates the dependence of the three-frequency NQRsignal of a powder of sodium nitrite at ω₊ on the nutation angle,γ_(N)B₁₀t_(p), for the serial pulse sequence shown in the inset (τ=0).

[0035]FIG. 5 illustrates the dependence of the three-frequency NQRsignal at ω₊ on the nutation angle γ_(N)B₁₀t_(p) for the simultaneouspulse sequence shown in the inset.

[0036]FIG. 6 illustrates the observed three-frequency spin-echo signal(symbols) as a function of the nutation angle γ_(N)B^(a) ₁₀t_(p) ^(a)for the parameters given.

[0037]FIGS. 7A & 7B illustrate the problem of acoustic ringing forsingle-frequency irradiation.

[0038]FIGS. 8A & 8B illustrate the problem of acoustic ringing forsimultaneous irradiation.

[0039]FIGS. 9A & 9B illustrate the elimination of acoustic ringing forserial irradiation.

[0040]FIGS. 10A & 10B illustrate steady-state pulse sequences forsimultaneous (FIG. 10A) and sequential (FIG. 10B) pulse irradiation. Thesequences are repeated for a total of n signal acquisitions. FIGS. 11A &11B illustrate relative SNR as functions of excitation pulse widths forsimultaneous (FIG. 11A) sequential (FIG. 11B) pulse irradiation wheret_(p+) are the pulse widths at 5.19 MHz and t_(p0) are the pulse widthsat 1.78 MHz. The sample is RDX.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

[0042] This invention provides a means for exciting and detectingthree-frequency pulsed nuclear quadrupole resonance (NQR). One of themain applications is in explosives and contraband detection by NQR. Forexample, the three-frequency technique helps to reduce the receiver deadtime in an NQR measurement, which can be particularly important if avery high-Q receiver coil is used. In addition, the three-frequencytechnique provides an even more material-specific signature than asingle-frequency NQR technique. In general, the three-frequencytechnique, as an extension of traditional NQR, is also useful in somecircumstances to better study the behavior and properties of certainmaterials. It is also noted that the three-frequency technique with aknown and appropriate material present can be used as a simple logicgate since the signal at the third frequency is only observed if both ofthe two excitation frequencies are present.

[0043] For a spin-1 nucleus such as ¹⁴N, there are three distincttransition frequencies as shown in FIG. 1. It is important to note thatin order to use a multiple-frequency technique, the nucleus to beexamined should have asymmetry parameter η not equal to 0 or 1, forwhich the three energy levels are distinct. Fortunately, the ¹⁴N foundin many explosives has a large asymmetry parameter. As an example, forRDX, η=0.62 and the NQR resonant frequencies are υ₊=5.2 MHz, υ⁻=3.4 MHz,and υ₀=1.8 MHz.

[0044] A major aspect here is the use of simultaneous excitation at twofrequencies to create the desired signal at a third frequency, and alsothe method and techniques for the creation of spin echoes and othersignals analogous to those obtained with standard multipulsesingle-frequency NQR methods. Echoes, for example, are highlyadvantageous for increasing the signal-to-noise ratio per unit time andthereby improving the detection. Multiple echos, such as in thespin-lock spin echo (SLSE) sequence, are a common way to significantlydecrease the acquisition times for weak signals.

[0045] Separating the excitation frequencies from the detectionfrequency allows the use of low Q transmitter coils to produce largebandwidth pulses while using a high Q detector coil. This will be anadvantage in super-Q detection of broad lines.

[0046]FIG. 2 shows a block diagram for the NQR detection system for apreferred embodiment of the present invention. The RF source (1), thepulse programmer and RF gate (1) and an RF power amplifier (1) areprovided to generate a train of RF pulses having a frequencycorresponding to one of the three NQR transition frequencies of the typeof explosive (e.g., all RDX-based explosives) or narcotic desired to bedetected. The coupling network (1) conveys this train of RF pulses tothe irradiating and detecting means (typically a coil). Similarly, theRF source (2), the pulse programmer and RF gate (2) and RF poweramplifier (2) are provided to generate a train of RF pulses having afrequency corresponding to a second frequency of the three NQRtransition frequencies. The coupling network (2) conveys this train ofRF pulses to the irradiating and detecting means (typically a secondcoil). Likewise, the RF source (3), the pulse programmer and RF gate (3)and RF power amplifier (3) are provided to generate a train of RF pulseshaving a frequency corresponding to the third frequency of the three NQRtransition frequencies. A coupling network (3) conveys this train of RFpulses to a third irradiating and detecting means (typically a thirdcoil). The coupling networks ((1), (2), and (3)) also conduct the signalto the receiver/RF detectors ((1), (2), and (3)) from the coils while aspecimen is irradiated with the train of RF pulses from all threesources. A central processing unit (CPU) controls the RF sources and thepulse programmers and RF gates. The CPU also processes the data andcompares the NQR signal with a predetermined threshold value. When thepredetermined threshold value is exceeded, an optional alarm isactivated in response to the comparison by the CPU. The receiver/RFdetectors, the RF power amplifiers, the pulse programmers and RF gates,the RF sources, and the CPU and the alarm may be contained in a consolewith only the coil and the coupling networks being outside of theconsole. The embodiment shown is one of the most general possibleapplications of three-frequency NQR.

[0047] Although in this embodiment the same means is used for bothirradiating the sample with the excitation radiation and detecting theNQR signal (e.g., one coil is used for both functions), this is not arequirement of the invention. Separate irradiation and detection means(e.g., separate irradiation and detection coils) may be employed ifdesired. For simplicity, common transmitter and receiver coils are usedfor each frequency in the three-frequency NQR, though separate sets ofcoils could be employed.

[0048] The free induction decay curve is a time domain curve. Byperforming a Fourier transform on this curve, a frequency domain NQRspectrum of the target species may be obtained.

[0049] Although a few preferred embodiments of the present inventionhave been shown and described, it would be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

[0050] The technique utilized according to the present invention is purenuclear quadrupolar resonance as taught in the previously mentionedBuess et al. patent. Three frequency excitation and detection may beperformed by combinations of any means known in the art, for example, asurface coil, such as a meanderline coil or a more conventional ‘volume’coil such as a cylindrical or rectangular solenoid, a toroid, or aHelmholtz coil. Pure NQR is typically performed in zero magnetic field:no magnet is required.

[0051] Obviously, many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the claims, the invention maybe practiced otherwise than as specifically described.

EXAMPLES

[0052] The experiment was carried out using ¹⁴N nuclei in a powdersample of sodium nitrite, NaNO₂, at room temperature. Given sodiumnitrite's asymmetry parameter of η=0.379, the level spacing is unequaland the transition frequencies are distinct (ω₊/2π=4.64 MHz, ω⁻/2π=3.60MHz, and ω₀/2π=1.04 MHz). The 8 gm sample was placed at the center of aprobe having three mutually orthogonal pairs of coils (see FIG. 3). RFpulses at ω₀ were produced by a Tecmag spectrometer, suitably amplified,and coupled into a Helmholtz coil oriented along y′, RF pulses at ω⁻were produced by a frequency generator and coupled to a Helmholtz coiloriented along x′, and the third pair of coils oriented along z′ wasused to detect the signal at ω₊. The strengths of the RF magnetic fieldswere measured using a small pick-up coil; B₁s as large as 2 mT were usedfor this experiment. It was shown that indirect mixing of thefrequencies resulted in only a negligible RF pulse at ω₊(B₁₊<1 μT).

[0053] Each set of coils was parallel tuned to the appropriate NQRfrequency using variable capacitors. This frequency-selective tuning andthe coils' mutual orthogonality provide more than 25 dB of isolationbetween the transmit and receiver coils so that RF leakage of ω⁻ and ω₀at the ω₊ receiver probe is significantly reduced. The RF leakage of ω⁻and ω₀ reaching the preamplifier is further reduced by analog filters.Therefore the dead-time could be significantly reduced. Receiverdead-times as low as 70 ps were observed, approximately five timessmaller than typical dead-times seen in single-frequency experiments.

[0054] After filtering, the NQR signal at ω₊ is heterodyned to ω⁻ bymixing with RF from the ω⁻ frequency synthesizer (see FIG. 3) so thatthe random phase introduced by the pulse at ω⁻ is removed. The NQRspectrometer then operates normally to detect the signal at ω₀, and allmodifications are external to the spectrometer. Approximately 120measurements were conducted using simple phase cycling to removeundesired transients after the RF pulses.

[0055]FIG. 4 shows the peak amplitudes from the Fourier transform of thefree induction decay (FID) at ω₊ in response to irradiation of thesample first by ω⁻, immediately followed by a pulse of the same lengthat ω₀. The nutation angles on the horizontal axis correspond to aconstant B₁₀ and variation in pulse length between 50 and 300 μs. Tofirst approximation, the relaxation occurring during the pulses wastaken into account by considering the signal at the same time after thebeginning of the second pulse. Also shown in FIG. 4 are thecorresponding theoretical predications using the independently measuredfields B₁₀ and B¹⁻, and the sensitivity of the receiver coil asdetermined by a single-frequency NQR experiment at ω₊. (There are noadjustable parameters.) Although the data agree fairly well withtheoretical prediction for smaller nutation angles (γ_(N)B₁₀t_(p)<2π),they deviate for larger nutation angles.

[0056] The amplitudes of the NQR signals at ω₊ in response tosimultaneous irradiation of the sample by RF pulses at ω₀ and ω⁻ agreewell with theoretical predications over the entire range of nutationangles studied. The symbols in FIG. 5 correspond to signals taken at thesame time after the center of the pulse, in order to take into theaccount relaxation occurring during the pulse length (60 to 500 μs). Byvarying the values of the magnetic fields B₁₀ and B¹⁻ we fit theoreticalpredictions to the experimental data. The best fits, shown as curves inFIG. 5, correspond to magnetic field strengths approximately 6% higherthan measured values, well within experimental error bars.

[0057] For a pair of simultaneous pulses separated by time τ, weobserved a spin-echo around t=τ after the end of the second pulse; seethe inset of FIG. 6. Phase cycling eliminated the FID from the secondpulse. The signal from t=τ onwards was Fourier transformed to compare itto a single-frequency experiment. The vertical axis in FIG. 6, wasnormalized to the maximum FID signal for a single-frequency experimentat ω₊. For a given pair of pulses τ was varied (from 5 ms down to lessthan 0.5 ms) and the signal amplitude was extrapolated back to τ=0 usinga linear fit to the data. A linear fit seems to characterize the datawell. The extrapolated amplitudes corresponding to τ=0 are plotted assquares in FIG. 6 as a function of nutation angle γ_(N)B₁₀t_(p) ^(a)(t_(p) ^(a) ranges from 180 to 420 μs for FIG. 6 data). For this datathe ratios of B₁s for the two sets of pulses were governed by B^(a)¹⁻/B^(a) ₁₀=tan(π/8), B^(b) ¹⁻/B^(b) ₁₀=tan(π/4), and t_(p) ^(b){squareroot}{square root over (B₁₀ ^(b2)+B¹⁻ ^(b2))}=t_(p) ^(a)°{square rootover (B₁₀ ^(a2)+B¹⁻ ^(a2))}. As can be seen in FIG. 6, the theoreticalpredictions of the spin-echo amplitude are in reasonable agreement withexperimental measurements for these optimal parameters.

[0058] The oscillating magnetic field of the RF pulse produces smallchanges in the physical dimensions of ferromagnetic materials (acousticringing) and conversely, these physical deformations produce changes ofmagnetization in the material which can then be detected as anartificial signal. The dashed line of FIG. 7A shows the resultingresponse from a magnetized paper clip to a single-frequency pulse atω₊/2π=4.64 MHz. For comparison, the response of an 8 g sample of sodiumnitrite NaNO₂ to the same pulse is shown as a solid line. Following thesame peak in frequency space (designated by a star FIG. 7B) it wasobserved that the response of the paper clip to the RF pulse is linearboth in the RF field strength and in the duration of the pulse, forpulse widths short compared to the time constant of the acousticringing, here ca 1 ms.

[0059] For simultaneous irradiation of the paperclip with the twofrequencies ω⁻ and ω₀ (3.60 M and 1.04 M a surprisingly strong acousticringing signal was observed at ω₊ as shown by the dotted line of FIGS.8A and 8B. However, for serial irradiation of the same paperclip firstwith ω⁻ then with ω₀ no acoustic ringing at ω₊ was observed as shown inFIGS. 9A and 9B. The pulse parameters used in FIGS. 8A, 8B, 9A, 9B areclose to the optimum needed for an NQR signal for each sequence (the NQRsignals are shown as solid lines). The fact that the acoustic ringing atω₊ appears for simultaneous, not for serial irradiation, implies thatthe deformation of the paperclip in response to the oscillating magneticfield contains a large nonlinear component. The response from thepaperclip was linear in the duration of the simultaneous pulse (notshown) and in the two field strengths B₁ and B₂ (shown in FIG. 8B). Thissuggests that the nonlinear component is proportional to the square ofthe total magnetic fields. Therefore the use of serial irradiation ofthe sample with ω⁻ and ω₀ and detection at ω₊ looks like a promisingmeans to detect a ¹⁴N NQR signal, without interference from acousticringing. Furthermore, as shown in FIG. 8A and FIG. 9A, the receiver deadtime is significantly reduced over that of a single-frequencyexperiment.

[0060] In order to increase SNR, multiple pulse sequences were usedwhere RF pulses are applied in a time scale that is shorter than thetime required for the spin system to return to equilibrium. Applicationof such steady-state pulse sequence with the three frequency NQR methodhas been demonstrated using RDX as the sample. As in the singleexcitation case, both simultaneous and sequential irradiation wastested. The pulse sequences are shown in FIGS. 10A & 10B. The RFexcitation used frequencies ω₊/2π=5.19 MHz, ω₀/2π=1.78 and detection wasat ω/2π=3.41 MHz. The excitation and data acquisitions were repeated fora total of n times and the resulting signals from the pulses wereaveraged together. Different combinations on non-alternating andalternating RF phases of 0 and π were used for the pulse pairs of t_(p+)^(a), t_(p+) ^(b), and t_(p0) ^(a), t_(p0) ^(b) with the beston-resonance SNR obtained using non-phase alternating pulses. Usingexcitation RF fields of B₁₊=0.26 mT and B₁₀=0.28 mT, signal amplitudeswere obtained as functions of the excitation pulse widths. The resulting2-dimensional plots of the relative SNR are shown in FIGS. 11A & 11B.Using the optimized pulse widths for the simultaneous and sequentialexcitation sequences, statistical measurements using a repetition of 200experiments using identical pulse parameters showed simultaneousexcitation to have SNR per unit time approximately two times larger thanthe sequentially excited sequence, and the overall sensitivity iscomparable to conventional single frequency multiple pulse sequencesused for detection of ¹⁴NNQR signal in RDX.

What is claimed is:
 1. An apparatus for examining a specimen by nuclearquadrupole resonance, comprising: a device for irradiating a specimenwith a first radio frequency pulse along an x-axis and a second radiofrequency pulse along a distinct y-axis and receiving along a distinctz-axis a signal from said specimen in response to said irradiation,wherein said radio frequency pulses and said signal are defined by thenuclear quadrupole resonance frequencies of the nucleus being examined.2. An apparatus as in claim 1, further comprising: (a) a unit forcomparing said signal to a predetermined threshold value; and (b) analarm for signaling when said signal exceeds said predetermined value.3. An apparatus as in claim 1, wherein said x-axis, y-axis, and z-axisare orthogonal.
 4. An apparatus as in claim 1, wherein said first andsecond radio frequency pulses are near to a ¹⁴N nuclear quadrupoleresonance frequency of a predetermined type of explosive or narcotic tobe detected.
 5. An apparatus as in claim 4, wherein said predeterminedtype of explosive or narcotic to be detected is selected from the groupconsisting of RDX-based explosives, HMX, PETN, TNT, ammonium nitrate,potassium nitrate, cocaine, and heroin.
 6. An apparatus for examining aspecimen by nuclear quadrupole resonance, comprising: (a) a firstgenerator for generating a first radio frequency pulse having a firstpredetermined frequency and a first radio frequency magnetic field; (b)a first device for irradiating a specimen with said first radiofrequency pulse and said first radio frequency magnetic field; (c) asecond generator for generating a second radio frequency pulse having asecond predetermined frequency and a second radio frequency magneticfield; (d) a second device for irradiating said specimen with saidsecond radio frequency pulse and said second radio frequency magneticfield; and (e) a detector for detecting a signal from said specimen inresponse to irradiating said specimen, wherein said radio frequencypulses are determined by the nuclear quadrupole resonance frequencies ofthe nucleus being examined and wherein said first device is a first coilabout an x-axis for irradiating said specimen about said x-axis and saidsecond device is a second coil about a distinct y-axis for irradiatingsaid specimen about said y-axis and said detector is a third coil abouta distinct z-axis for receiving said signal about said z-axis.
 7. Anapparatus as in claim 6, wherein said radio frequency magnetic fieldsare orthogonal.
 8. An apparatus for examining a specimen by nuclearquadrupole resonance, comprising: (a) a first coil about a first axis;(b) a second coil about a second axis; (c) a third coil about a thirdaxis; (d) a first power source connected to said first coil forgenerating a first radio frequency pulse having a first predeterminedfrequency and a first radio frequency magnetic field; (e) a second powersource connected to said second coil for generating a second radiofrequency pulse having a second predetermined frequency and a secondradio frequency magnetic field; and (f) a first detector connected tosaid third coil; wherein said first and second predetermined frequenciesare defined by the nuclear quadrupole resonance frequencies of thenucleus to be examined.
 9. An apparatus as in claim 8, wherein saidcoils are orthogonal.
 10. An apparatus as in claim 8, furthercomprising: (a) a fourth coil about same axis as said first coil; (b) afifth coil about same axis as said second coil; (c) a sixth coil aboutsame axis as said third coil; (d) a second detector connected to saidfourth coil; and (e) a third detector connected to said fifth coil. 11.An apparatus as in claim 10, wherein said axes are orthogonal.
 12. Amethod for detecting a class of target species containing quadrupolarnuclei in a specimen by nuclear quadrupole resonance, comprising: (a)generating a first radio frequency pulse having a first predeterminedfrequency; (b) irradiating said specimen with said first radio frequencypulse; (c) generating a second radio frequency pulse having a secondpredetermined frequency; (d) irradiating said specimen with said secondradio frequency pulse; and (e) detecting a nuclear quadrupole resonancesignal at a third frequency in response to irradiating said specimen;wherein said pulses and nuclear quadrupole resonance signal are definedby the nuclear quadrupole resonance frequencies of the nucleus beingexamined.
 13. A method as in claim 12, wherein said first and secondradio frequency pulses are irradiated simultaneously.
 14. A method as inclaim 12, wherein said first and second radio frequency pulses areirradiated sequentially.
 15. A method as in claim 12, wherein said firstpredetermined frequency is set to equal said third frequency and as aresult said third frequency now equals what was the value of said firstpredetermined frequency.
 16. A method as in claim 12, wherein said firstand second radio frequency pulses are near to a ¹⁴N nuclear quadrupoleresonance frequency of a predetermined type of explosive or narcotic tobe detected.
 17. A method as in claim 16, wherein said predeterminedtype of explosive or narcotic to be detected comprises RDX-basedexplosives.
 18. A method as in claim 15, wherein said firstpredetermined frequency is irradiated simultaneously as said secondpredetermined frequency.
 19. A method as in claim 15, wherein said firstpredetermined frequency is irradiated sequentially with said secondpredetermined frequency.
 20. A method as in claim 12, wherein said firstpredetermined frequency is irradiated both sequentially andsimultaneously with said second predetermined frequency.